The use of high-energy-product permanent magnets (also known as rare earth magnets) in electrical machines is becoming more prevalent, especially in large-diameter low-speed permanent magnet motors and generators. Due to the high magnetic forces that are produced by these magnets and the highly brittle nature of the magnetic material and its susceptibility to corrosion if not suitably protected, their installation to form permanent magnet arrangements presents some difficulties.
A number of arrangements have been proposed which facilitate the installation of permanent magnets, and especially high-energy-product permanent magnets, into electrical machines and which at the same time provide protection for the brittle and corrodible permanent magnet material. GB 2438443, WO 2008/046780 and EP 1990811 all propose arrangements in which one or more permanent magnets are mounted on a magnet carrier. A cap or cover is fixed to the magnet carrier by mechanical fixings, welding or by other suitable means. The magnet carrier, with its permanent magnets and integral cap or cover, is finally fixed to the rotor of the electrical machine to form a permanent magnet rotor arrangement.
A particular disadvantage of these arrangements is that the cap or cover must be fixed to the magnet carrier during assembly of the permanent magnet arrangement. This is an unwelcome step which increases the complexity of the assembly process. It would, therefore, be desirable to provide an improved permanent magnet arrangement for an electrical machine which provides good mechanical and environmental protection for the permanent magnets and which can be easily assembled and installed.